A. Field of Invention
This invention pertains to a method and apparatus for testing audio programs, and more particularly to a means for determining rapidly the proper phase and polarity and other characteristic parameters of stereo audio signals of said programs through visual inspection. The method and apparatus include generating a unique test signal which can be used to generate an image indicative of various characteristic parameters associated with test signal and the related.
B. Description of the Prior Art
Standard recordings of stereo audio programs, for example, master tapes, digital audio files, and analog or digital video files, consist of stereo signals having certain characteristic parameters, such as phase and polarity. These characteristic parameters must be preserved while of the programs are copied or transmitted to a remote location to insure that they can be replayed as accurately as possible. Preserving these characteristic parameters also insure that the artistic rendition of the programs are not compromised by errors.
The usual technique used to verify these characteristic parameters consists of inserting test signals (typically sinusoidals) at the beginning of each audio program. The recorded or transmitted program is then checked by displaying the test signals on a graphic analyzer, such as an X-Y oscilloscope. More specifically, the test signals on the left channel are connected to the X input of the oscilloscope (corresponding to the horizontal axis), and the test signals on the right channel are connected to the Y input of the oscilloscope (corresponding to the vertical axis). The image generated by the oscilloscope is then inspected by a skilled operator.
A disadvantage of this method is that the operator needs a high level of skill and experience to identify and correct any recordal or transmission problems since they are not patently obvious. In addition, sinusoid test signals are useful to identify some but not all of the problems that can occur while a program is recorded or transmitted. For example, the method is not capable of identifying an absolute phase reversal, wherein the phase of the signals of both the left and the right channels are reversed. Moreover, the existing technique cannot be used to determine whether the program has been copied improperly or otherwise duplicated using some encoding and decoding non-linear schemes
In order to assure uniformity and accuracy in the copying and transmission of stereo programs, various national and international organizations have promulgated standards for stereo audio programs. Some of these standards define the proper phase and polarity parameters of the stereo audio signals on each channel. However, the standards do not specify any techniques for determining, or testing that recorded or transmitted programs include stereo signals with the correct phase and polarity.